Hydrocarbon solid oxide fuel cells (SOFCs) have increasingly attracted much attention worldwide due to their direct conversion of chemical energy in feeds, especially feeds manufactured from fossil resources, into electrical power with high efficiency and low impact on the environment [1]. Conventional hydrocarbon SOFCs use oxygen ion conducting electrolyte, and completely oxidize hydrocarbon feed to CO2 gas and H2O at the anode [2]. Several anode catalysts including ceria and perovskite containing materials have been reported for hydrocarbon oxidation in solid oxide fuel cells with oxygen ion electrolytes including yttrium stabilized zirconia (YSZ) [3].
When compared with conventional oxygen ion electrolytes, proton conductors have higher ionic conductivity due to the lower activation energy for proton conductivity at low to intermediate temperatures, offering potential for operation of proton-conducting SOFCs with higher performance, longer stability and lower cost, each of which is very important for realizing broad commercialization of SOFCs [4]. However, the reactions of hydrocarbons in the anode compartments of proton-conducting SOFCs differ from those in oxide ion conducting SOFCs because there is no oxygen source available to for deep oxidation of the hydrocarbon feed. In principle, if the anode has the capability to readily dehydrogenate hydrocarbons, proton-conducting SOFCs have the potential to convert the feed to electrical energy and dehydrogenated chemicals since the protonic electrolyte primarily or solely conducts protons from the anode to the cathode. Therefore, the SOFC can also serve as a dehydrogenation membrane reactor, and operate as a fuel cell membrane reactor to co-generate power and a dehydrogenated product.
Hydrocarbons also are important feedstocks for the chemical industry. For example, ethylene, which usually is obtained in commercial quantities via steam cracking of ethane or other hydrocarbon feedstocks, is a major intermediate for production of polymers and petrochemicals. In the ethane steam cracking process a significant amount of ethane feed is burned to provide energy for this high endothermic dehydrogenation reaction. In order to reach high reaction temperature significant amounts of GHG are also emitted. Alternative methods, in particular oxidative dehydrogenation of ethane to ethylene, have been intensively researched. During ethane oxidative dehydrogenation substantial amounts of ethane unavoidably are deeply oxidized to CO2 and the chemical energy from the conversion of hydrogen is not easily recovered as high grade energy [5]. Further, oxidative methods may also produce acetylene, which is very detrimental to manufacture of polymers as it poisons the catalysts and so must be removed to form high purity ethylene feed, an expensive process [6].
In contrast, electrochemical dehydrogenation of ethane to ethylene in proton conducting SOFC reactors is potentially more selective than oxidative processes, allows recovery of high grade energy, and generates little or no pollutants [7, 8]. To achieve a high reaction rate and high current density, SOFCs are operated at the maximum sustainable temperature. Electrochemical oxidative dehydrogenation of alkanes to alkenes in proton-conducting polymer membrane fuel cells at temperatures lower than 155° C. [9], effects low conversion of alkane and low current densities, as there is low catalytic activity for alkane dehydrogenation and low proton conductivity under those conditions. Therefore it is desirable to develop materials and processes for operation of proton conducting fuel cells at high temperatures such as proton conducting solid oxide fuel cells.
To date, very few anode catalysts have been investigated for conversion of hydrocarbon feeds in proton conducting solid oxide fuel cells. In prior art fuel cells [7, 8, 10] Pt was used as the active anode catalyst in ethane solid oxide fuel cell membrane reactors and obtained good power density and ethylene selectivity. However Pt is expensive and, over time, is poisoned by carbon deposition at high fuel cell operating temperatures. Therefore it is desirable to develop stable anode catalysts for conversion of hydrocarbons at high rates in SOFCs, and that they be more resistant than Pt to formation of carbon deposits.